1. Field of the Invention
The present invention relates to a light guide unit for use in a liquid crystal display device in which a polarized component of light is enhanced and a liquid crystal display device which is provided with such light guide unit. Particularly, this invention relates to a light guide unit for efficiently converting the light from a light source to a polarized light and a liquid crystal display device having means for efficiently directing the polarized light emitted from such light guide unit to a liquid crystal cell.
2. Description of the Related Art
A liquid crystal display device is conventionally observed by directing polarized light to a liquid crystal cell to cause the polarization plane to be rotated depending on the condition of the cell for passage through a polarizer plate. A light source of the polarized light is placed in the back of the liquid crystal plate and thus is called a "back light". For obtaining such polarized light wave, a non-polarized light was conventionally incident to a polarizer plate and either one of the polarized components; i.e., S component and P component, was absorbed.
Assuming that a plane defined by a light incident to a point of incidence on a surface is an incident plane, a polarized component parallel to the incident plane is called a P component while a component perpendicular to the incident plane is called an S component. Therefore, more than 50-percent of an incident light was not effectively utilized in principle and an actual measurement shows that about 58-percent of the incident light is absorbed.
Further, a light dispersing sheet having printed dots was typically used in addition to a polarization device for obtaining polarized light by absorbing a polarized component in a conventional Liquid Crystal Display (LCD) device, and this makes an additional 20-percent of the light unavailable.
In FIG. 1, a LCD module 100 of a conventional LCD device is shown. The light emanating from a light source 101 transmits through a light guide plate 102 having 96% transmittance, a dispersion sheet 103 having 80% transmittance, a lower polarizer plate 104 having 42% transmittance, a glass substrate 105 having a numerical aperture of 40%, a color filter 106 having 30% transmittance, and an upper polarizer plate 107 having 90% transmittance, resulting in an actually available light intensity which is 3.5% of the light generated in the light source 101. This greatly prevents the energy from being utilized efficiently.
A back light system of a high intensity for use in a low power consumption LCD device is especially desired because it is an important objective in a portable personal computer to assure a longer usable time with a given capacity of a battery and the power consumption of a back light 108 is a major percentage of total power consumption.
Also, the light energy absorbed in the lower polarizer plate 104, etc., is converted to heat energy which contributes to degradation of parts of the LCD device. Particularly for a liquid crystal material of STN (Super Twisted Nematic) type in which the display quality is degraded by heat, it is an important objective to reduce such heat generation. As seen from FIG. 1, 66.4% of the light energy is converted to heat energy by the light absorption in the lower polarizer plate 104 and the dispersion sheet 103 (this is 69% of heat generation by the light energy).
In order to solve such problems, the applicant of this application filed Japanese patent application no. 9-249139 relating to a method of improving the efficiency of light utilization in obtaining a polarized light by making available for use at least a part of a polarized component which had not been utilized. The principle of this method is shown in FIG. 3.
Light from a fluorescent lump CFL which is a light source is incident to the end surface of a laminated light guide plate unit via a reflecting mirror and a collimator. It propagates through the layers of the light guide plates, and arrives at the other end surface which is cut in an angle. The incident light is partly reflected at the other end surface with the rest being transmitted therethrough. The polarization plane of the light transmitting through the end surface is rotated by a quarter wave length plate placed thereunder and reflected by a reflecting plate placed under the quarter wave length plate for reentrance to layers of the light guide plate again through the quarter wave length plate as a P component.
The P component reentering the light guide plates is incident to the interface with an adjacent light guide plate layer. The angle of incidence of the light on the interface is the Brewster angle (to be described later in detail). Therefore, all the P component and a part of the S component of the light incident to the interface transmit through the interface with the rest of the S component reflected back to the quarter wave length plate and the reflecting plate. The light reflected again by the reflecting plate is again directed to the interface after being converted to a P component by the quarter wave length plate where all the P component and a part of the S component, if any, transmit with the rest being reflected.
The light reflected here is reflected repeatedly in a similar manner and a light converted to a P component for each reflection transmits through the interface. As such, the light guide unit ultimately emits a large portion of the light from the light source as a P component. The polarized light is emitted in the direction largely deviated from the normal to the front. A prism sheet for redirecting the light to the front toward the liquid crystal cell is used. The polarization can be further improved by placing a further polarization plate on the prism sheet.
Because the reflectance and transmission characteristics are different between the S component and the P component, the light transmitting through the interface and the light reflected by the interface have different polarization components. To explain the principle of operation of this invention, a change of polarization components of the light in transmitting through or reflecting from the interface between materials of different indices of refraction is described with reference to FIGS. 4, 5 and 6.
In FIG. 4, when light 204 reaches an interface 203 between two materials 201 and 202 having different indices of refraction n.sub.1 and n.sub.2, respectively, a part of the light 205 is reflected when the angle of incidence .phi..sub.1 is less than a critical angle while a part of the light 206 transmits through the interface. Assuming that a plane defined by a light incident to a point of incidence on a surface is an incident plane, the incident light 204 is divided into a P component parallel to the incident plane and an S component perpendicular to the incident plane.
Modifying Maxwell equation for a dielectric material, the transmittance of the polarized components P and S are given by; EQU Tp=sin (2.phi..sub.1).times.sin (2.phi..sub.2)/(sin.sup.2 (.phi..sub.1 +.phi..sub.2).times.cos.sup.2 (.phi..sub.1 -.phi..sub.2)) EQU Ts=sin (2.phi..sub.1).times.sin (2.phi..sub.2)/(sin.sup.2 (.phi..sub.1 +.phi..sub.2) EQU n.sub.1 .times.in(.phi..sub.1)=n.sub.2 .times.sin (.phi..sub.2)
where
Tp: transmittance of P component (1-reflectance Rp) PA1 Ts: transmittance of S component (1-reflectance Rs) PA1 Rp reflectance of P component (1-transmittance Tp) PA1 Ts: reflectance of S component (1-transmittance Ts)
.phi..sub.1 : incident angle of light PA2 .phi..sub.2 : exit angle of light PA2 n.sub.1 : index of refraction of material 201 PA2 n.sub.2 : index of refraction of material 202
or it is known that; EQU Rp=((n.sub.1 / cos .phi..sub.1 -n.sub.2 / cos .phi..sub.1 +n.sub.2 / cos .phi..sub.2)).sup.2 EQU Rs=((n.sub.1 .times.cos .phi..sub.1 -n.sub.2 .times.cos .phi..sub.2)/(n.sub.1 .times.cos .phi..sub.1 +n.sub.2 .times.cos .phi..sub.2)).sup.2
where
The reflectance of the P polarized component and S polarized component vary depending on the incident angle .phi..sub.1 and the exit angle .phi..sub.2 as shown in FIG. 5 and FIG. 6, and differ from each other even in a same incident angle .phi..sub.1 (reflectance/transmittance characteristics are different between S and P polarized components).
For example, when the light proceeds from an acrylic material having an index of refraction of 1.49 to air which has an index of refraction of 1.00 (FIG. 6), the critical angle in which a total reflection takes place is 42.1-degrees. If the light is incident at 40-degrees which is less than the critical angle, the exit angle .phi..sub.2 will be 77.8-degrees according to Snell's law. Substituting the above equation of Rs and Rp with this, the reflectance for the S component is 35.69% while the reflectance for the P component is 7.98%.
It should be clearly understood from the above description referring to FIGS. 4 to 6 how the polarized components of the light are transmitted and reflected in the interface in this invention.
It is understood from the above-described principle that it is important for the layers of the light guide to be laminated in multiple layers to cause the unnecessary S component to be reflected back each time the light reaches the interface between the layers and to be returned as a P component for transmitting through the interface thereby improving the efficiency of converting the light emitting from the unit eventually to a P component.
However, it is disadvantageous to laminate too many layers from the view point of the efficiency of utilizing the energy of the light source because each layer invites some loss of light. In addition, the increased number of laminated layers would result in the increase of the thickness of the entire unit even if a thin layer is used. The increase of the thickness would also invite an increase of the weight. It is the most important objective for a portable information processing device, such as a notebook computer, to decrease the power consumption of its battery as well as the thickness and the weight of the entire unit as much as possible.